Production of plutonium fluoride from bismuth phosphate precipitate containing plutonium values



- United States Patent 7 PRODUCTION OF PLUTONIUM FLUORIDE FROM 'BISMUTHPHOSPHATE PRECIPITATE CONTAIN- ING PLUTONIUM VALUES No Drawing. FiledAug. 31, 1948, Ser. No. 47,142

1 Claim. c1. 23-145 This invention is concerned with an improved methodof separating plutonium from contaminating elements normally associatedwith plutonium in neutron-irradiated uranium.

The Word plutonium, as hereinafter used in the specification and claimsrefers to the element of atomic number 94 and to the compounds thereof,unless the context indicates clearly that plutonium is referred to inits metallic state.

Natural uranium is composed of three isotopes; namely U U and U thelatter being in excess of 99% of the Whole. When U is subjected to theaction of slow or thermal neutrons, a fourth isotope, U is producedhaving a half-life of twenty-three minutes and undergoing beta decay toNp which in turn decays with a half-life of two and three-tenths days toyield plutonium. In addition to the formation of 94 3 there aresimultaneously produced other elements of lower atomic weight knownas'fission fragments. These fission fragments are composed of elementshaving atomic numbers from about 32 to 64. The elements of this group,as originally produced, are considerably overmassed and undercharged andhence are highly unstable. By emission of beta particles accompanied bygamma radiation, these elements transform themselves into isotopes ofthese various elements having longer half-lives. The resulting materialsare commonly known as fission products.

Various radioactive fission products have half-lives ranging from afraction of a second to thousands of years. Those having very shorthalf-lives my be substantially eliminated by aging the material for areasonable period before handling. Those with very long halflives do nothave sufficiently intense radiation to endanger personnel protected bymoderate shielding. On the other hand, the fission products havinghalf-lives ranging from a few days to a few years have dangerouslyintense radiations which cannot be eliminated by aging for practicalstorage periods. These products are chiefly radioactive isotopes of Sr,Y, Zr, Nb, Te, I, Cs, Ba, La, Ce, and Pr.

It may be readily seen that plutonium, as produced by the processgenerally set forth above, is contaminated with considerable quantitiesof uranium and fission products. In fact, the plutonium constitutes onlya very minor port-ion of the irradiated mass, i.e., less than 1%thereof. In view of such a low concentration of plutonium intheirradiated metal and the highly radioactive character of the fissionproducts present, it becomes apparent that the procedure employed to.recover ice that element must be highly efiicient in order to be at allpracticable.

There have been devised a number of procedures for the removal andconcentration of plutonium from extreniely dilute solutions thereof. Ingeneral, such methods involve the formation in said dilute solutions ofvarious insoluble compounds capable of carrying plutonium in one of itsvalence states. The carrier precipitate and plutonium thus obtained arethen dissolved and the plutonium converted to another valence state, inwhich state it is soluble in the presence of said carrier. The carrieris then re-precipitated from the new solution, thus removing the fissionproducts present in this solution, but leaving the soluble plutonium insolution. Thereafter, the plutonium may be separated from the solutiondirectly as a plutonium compound or the plutonium may be converted tothe valence state in which it is insoluble with the aforementionedcarrier and may be removed from the solution with that carrier. Thiscarrier precipitate may again be dissolved and the plutonium purifiedfurther, if considered necessary or desirable, by repeating the abovecycle. A carrier precipitate which carries plutonium is usually referredto as a product precipitate, while a carrier precipitate which removeselements other than plutonium, leaving the plutonium in the solution isusually referred to as a by-product precipitate.

The precipitation method of separation is usually divided into foursteps. The steps are: Extraction, in which the plutonium is separatedfrom the uranium and most of the fission products; decontamination, inwhich the plutonium is separated from theremaining fission productsconcentration, in which the ratio of carrier precipitate or containingsolution to plutonium is greatly reduced; and isolation, in which thefinal plutonium compound is precipitated directly from solution.

One of the most important plutonium precipitation processes is thebismuth phosphate process. In this process the neutron-irradiateduranium mass, after suitable aging, is dissolved in nitric acid to forma uranyl nitrate hexahydrate solution. A bismuth phosphate .precipitateis then formed in and separated from this solution, carrying with itplutonium in a valence state less than 5 and fission products which arephosphate-insoluble, particularly zirconium and niobium. Thisprecipitate is then redissolved in nitric acid, the plutonium convertedto the hexavalent state, and a by-product bismuth phosphate precipitatecarrier formed in, and separated from, this solution, carrying with itthe phosphateinsoluble fission products. The plutonium contained in thesupernatant solution is then reduced to the tetravalent state andcarried from the solution with a bismuth phosphate carrier precipitate.This precipitate is then dissolved in nitric acid, the plutoniumoxidized to the hexavalent state, and a bismuth phosphate by-productcarrier precipitate separated from the solution. Following this, theplutonium is reduced to the tetravalent state and separated from thesolution with a lanthanum fluoride carrier. This carrier of lanthanumfluoride is metathesized with an alkali metal carbonate or hydroxide,the resulting mixed lanthanum and plutonium hydroxides dissolved, andthe plutonium precipitated directly from the solution without a carrier.

While this type of plutonium separation process is highplutonium in apure state.

dispose of safely.

ly efiicient in the separation 'of'plutoniumfrom contaminants normallyassociated with plutonium in neutronirradiated uranium, particularly theradioactive fission products, there are certain disadvantages andinconveniences inthe use of this process. The PIOCBdIJl'B'lS obviouslycumbersome and time consuming, sinceitrequires a number ofprecipitationsteps in order to obtain the of reagents and the process leavesa largequantity of radioactive and waste lay-products, which are diflicult toprocess forseparating plutonium from contaminants with a lesser numberof steps than the precipitation process would have great utility.

It is an object of this invention to provide a convenient and efficientmethod of recovering plutonium from impuritiesassociated therewith in aneutron-irradiated uranium mass.

It is a further object of our invention to provlde a method 'that'willeffectively separate plutonium from It will be readily apparent thatany,

verted to the tetravalent plutonium fluoride, which however is notvolatile below 600 C. The mass is then treated with fluorine attemperatures of about 290 C.

or above and preferably betweenabout 400-500 C.,

whereupon the lower plutonium fluoride is converted to p the highervolatile fluoride (probablythe pentaor hexa- It requires a vast amountfluoride), volatilized andthuseflectively separated from the lessvolatile impurities in the mass, particularly radioactive zirconium theplutonium fluoride is condensed at below 290 C. The bismuth. fluorideresidue is dis- :tilled off at about the same temperature as theplutonium fluoride and so other procedures must be used in separatingthe plutonium and bismuth fluorides obtained by this process. Thehydrogen fluoride and fluorine used in this process should be free fromoxygen and water vaporin order to avoidthe complications caused byoxyfluoride fission products and other impurities normallyassociated Itherewith, in'a small number of operations.

Additional objects of our invention will be apparent as the presentdescription proceeds,

Broadly, this invention comprises the hydrofluorination The carrier massis then treated with anhydrous fluorine at elevated temperatures,whereby the plutonium is oxidized and volatilized as itshigherfluoridesand thus separated from the less volatile constituents of the mass.

In accordance with the preferred embodiment of ourinvention,-neutron-irradiateduranium is aged for from, I sixty to ninetydays, thereby substantially converting the I neptunium present toplutonium'and permitting the most active fissionproducts. to decay tomore stable isotopes. The uranium mass is then dissolved in a suitableacid, for example, nitric acid. The uranium is normally present. in suchsolution as the uranyl ion and the plutonium as the tetravalent ion.However, to insure that the plutonium is present in a valence state lessthan +5, the solution may be treated with a reducing agent, such asformic acid or sodium nitrite in the presence of sulfuric acid, wherebythe dissolved plutonium is converted substantially completely to thetetravalent state. A bismuth phosphate precipitate is then formed in theuranyl nitrate hexahydrate solution and separated therefrom. Thisbismuth phosphate precipitate carries with it the plutonium present andthe fission products which are phosphate-insoluble. The uranyl ion and asubstantial portion of the fission products are phosphate-soluble and soremain in the solution after the separation. Since the uranium isgreater than 99% of the total mass, this results in a great reduction inthe amount of foreign products which contaminate the plutonium. Thefission activity is also reduced by this step by about 87%. The 13% offission product activity carried with the bismuth phosphate isattributed largely to zirconium and niobium isotopes. The bismuthphosphate plutonium carrier precipitate thus obtained is dried andthereafter placed in a suitable fiuorination reactor.

The dried precipitate is then subjected at elevated temperatures, forexample at a range of 500-600 C., to the action of hydrogen fluoride,preferably in the anhydrous state, whereupon the niobium present isconverted to the volatile niobium fluoride and readily separated fromthe remainder of the solid mass. Any tracesof other fission products,the fluorides of which have high volatility, may also be separated atthis point. These may include As, Se, Br, Sb, Te, and I. The radioactiveinert gases, Kr and Xe, will also be completelyremovedby this step.Plutonium present in the mass will be substantially conformation.

, Our invention may be further illustrated by the followingspecificexample. p p p Example To 25 ml. ofa nitric acid solution ofneutron-irradiated uranium,which haspreviouslybeen treated with sodiumnitrite in the presence of H 50 to insure that the plu- 'tonium presentis in an oxidation state of less than +5,

is. added 1 g. of bismuthnitrate and the solution made Theresultingmixture is agitated for 10 minutes at 75 C. and bismuthphosphateplutonium carrierprecipitatethus. formed is separated from thesolution by centrifugation. tained. which contains approximately 5.0 mg.of plutonium together with fission products is next placed in a dryingoven at a temperature of about 100 C. and allowed to Thereafter theremain until substantially anhydrous. dry bismuth'phosphate plutoniumcarrier precipitate is placed ina nickel tube fluorination reactor ofconven a period of two hours at 500 C. Under these conditions theplutonium present is'converted to plutonium tetrafluoride which,however, is not volatile below 600 C. and'thus remains with the mass.Niobium and other volatile impurities, however, are volatilized by thisstep. Anhydrous fluorine is next introduced at a rate of about 0.3 literperhour at a temperature of 525 C. under which conditions the plutoniumtetrafluoride is transformed to a volatile fluoride and is collected ina suitable-receiving vessel at a temperature below 275 C. The bismuthfluoride formed by this fluorination step accompanies the plutonium andis collected with it. The remaining fission products whichh avefluorides less volatile than the higher plutonium fluorides, are left asa residue inthe reaction chamber. The plutonium recovered in this mannerrepresents approximately of that originally present in the uranylnitrate hexahydrate solution.

It will be apparent to those skilled in the art that the method ofrecovering plutonium, as generally set forth above, provides a simpleand practical procedure for the separation of plutonium from radioactivefission products. ilt will also be noted that the radioactive fissionproducts are recovered in a concentrated and easily handled form.

While this invention has been illustrated by certain applications, it isnot desired to be specifically limited thereto, since it is manifest tothose skilled in the art to which the present invention is directed,that it is susceptible to numerous alterations and modifications withoutdeparting from the scope thereof. While it is illustrated as a step in acontinuous process for the separation of plutonium fromneutron-irradiated uranium, the process may also be suitably used forthe separation of plutonium from any contaminant which forms a fluoridehaving a boiling point diiferent than that of a higher fluoride ofplutonium.

What is claimed is:

A process of producing a mixture of bismuth and plu- The solid residuethus obtonium fluorides free from fission products from a hismuthphosphate carrier precipitate containing plutanium Values in a valencestate of less than +5 and fission product values, comprising drying saidprecipitate; subjecting said precipitate to the action of hydrogenfluoride at a temperature of between 500 and 600 C. whereby theplutonium is converted to plutonium tetrafluoride and the fissionproducts to the fluorides some of which volatilize away from a residuecontaining said bismuth phosphate, plutonium tetrafluoride andnonvolatile fission product fluorides; contacting the residue withanhydrous fluorine at a temperature of between 400 and 500 C.

ly nonvolatile fission product fluorides; and condensing said mixture ofplutonium fluoride and bismuth fluoride 5 by cooling to below 290 C.

References Cited in the file of this patent UNITED STATES PATENTS Werneret a1. Jan. 1, 1957 2,815,265 Werner et a1. Dec. 3, 1957

